And what is the difference between the nebula in which stars are born and the clouds that we also observe on Earth?
The fundamental difference is in WHAT generates gas and dust aggregation in space nebula. When we talk about the birth of stars, we are referring to interstellar clouds that can be perturbed by an external event (e.g. the explosion of a nearby supernova, or even the close passage of other stars), and thus can be found with a large amount of mass moved within a limited space. If this mass collected in a certain volume exceeds a critical value (known as “Jeans mass”), then the internal pressure of the gas is not able to counteract the gravitational collapse of the cloud, which at this point will occur. The gas will then be collected more and more towards the regions of the nebula with the highest mass content, with density and temperature increasing until the first hydrogen nuclear burning reactions are triggered … a star is born! Now, in this case we are talking about interstellar clouds with a mass of the order of tens of solar masses (and a solar mass is equal to about 300 thousands Earths!). In these conditions, gravity is really effective and determines the structure of the nebula, and it is in fact what causes the collapse of the cloud towards the higher density areas.
This is also the main difference between space nebula gases and clouds we are familiar with in everyday life. All the clouds in the sky put together would not reach one millionth of the mass of the Earth. In such conditions, the gravitational interaction is of course present, but completely negligible, and in fact in such conditions a gravitational collapse will never occur. There are in fact other more important forces at play, such as chemical bonds for example, or the thermal collisions between the atoms that make them up, atmospheric streams… all these factors will determine the progressive movement and diffusion of gas and clouds in the environment, but as already mentioned, no collapse. In fact, we are talking about numbers well below the critical ones that determine the birth of the stars … “cosmic numbers” indeed!
References:
Jeans James Hopwood, 1902 I. “The stability of a spherical nebula”; Philosophical Transactions of the Royal Society of London. Series A.
